Method of making sponge rubber



7 Plmhd 1 29, 1943 METHOD OF MAKING SPONGE RUBBER William 1. Clayton,Mishawaka, Ind., assignor to United States Rubber Company, New York,

a N. Y., a corporation or New Jersey No Drawing. Application July 2,1941,

' Serial No. 400,799

4 Claims.

This invention relates to a method ofmakins sponge rubber and moreparticularly to a method of making sponge rubber from foamed latexhaving a high content of filler material.

Sponge rubber has been produced by convert ing latex into a foam,shaping the foam and coagulating the latex in the foam, as by gelling,to form sponge rubber. There are many known methods of preparing stableuncoagulated latex foams which after shaping may be readily convertedinto sponge rubber. For example, air may be bubbled into a body of latexcontaining a foam stabilizing agent, such as soap, and the resultantfoam may be agitated to subdivide the bubbles and form a stiffer foam.The air may be beaten into a compounded latex by means of a suitablydesigned apparatus similar to a cake mixer or egg beater. The latex maybe foamed by means of a gas generated in situ, either by thedecomposition of a chemical compound such as hydrogen peroxide, or bythe interaction of two or more chemicals to form a gas as by thereaction between a carbonate and an acid, or by the reaction between anactive metal and a base, as the reaction between powdered magnesium andpotassium hydroxide. The latex may be foamed by applying a vacuum to acontainer containing a latex which is capable of being frothed underreduced pressure by escape of gas dissolved in the latex or byproduction of gas in the latex as by chemical decomposition or heating.A latex compound may be foamed by impregnating it with a gas underpressure such as nitrous oxide or carbon dioxide and subsequentlyreleasing the pressure.

The latex foamed by any of these known methods may be shaped, as bypouring into molds having the proper size and configuration, followingwhich the latex in the foam may be coagulated to form sponge rubber bycausing the foam to gel or set. If desired, the foam may be gelled inthe apparatus in which it is formed.

,The gelling of the latex foam may be brought about in a variety ofways. There may be added to the latex, either before or after it isfoamed, certain heat-sensitizing agents, which act to cause a latexcompound to coagulate or gel when it is heated above some criticaltemperature. Among such known heat-sensitizing agents are the diandtri-valent metal salts, complex zincammonia salts, ammonium salts ofstrong acids, certain aryl substituted guanidines, easily hydrolyzableesters, ammonium persulfate,'and the salts of fluosilicic acid. Certainsensitizing agents. notably the alkali metal sillco fluorides,

possess the useful property of causing a latex foam to gel at roomtemperature after a lapse of time which is inversely dependent upon theamountof sensitizer added. After gelling of the foam, at room orelevated temperature, the thus formed sponge may be partially orcompletely vulcanized in the mold or other shaping apparatus, removedtherefrom, and dried.

It has been found in previous practice that the addition of any largeamount of inert filler material to the'latex compound so reduces the wetcoagulum strength of the gelled foam that even after vulcanizing it isextremely difilcult to remove from the molds or other shaping apparatuswithout tearing. Further, the addition of more than about 20 parts offilter material per 100 parts of rubber unduly reduces the flexingresistance of the finished sponge article.

The present invention comprises a treatment of the latex before foamingthat considerably increases the wet coagulum strength of the foam aftergelling, and increases the flexing resistance of the finished sponge. Bythe present inven tion, I have been able to make sponge rubber articlescontaining 200 or more parts of filler material per 100 parts of rubberthat have sufflcient wet coagulum strength to allow the gelled sponge tobe readily removed from the molds without tearing, and that haveadequate flexing resistance in the finished article for various uses.Further, by virtue of the present preliminary treatment of the latex, Ihave been able to make sponge rubber articles containing 10 to 20 partsof filler material that are as good, so far as the essential qualitiesare concerned, as unloaded sponge rubber articles made in the samemanner but without the preliminary treatment of the latex, and I havebeen able to make sponge rubber articles containing 50 parts of filtermaterial per 100 parts of rubber that are as good as sponge articleswith 20 parts filler material per 100 parts rubber made in the samemanner but without the preliminary treatment of the latex according tothe present invention.

In carrying out the present invention the latex is heated in thepresence of at least .5 part of an aldehyde, preferably formaldehyde,per 100 parts of latex Solids at a temperature above 100" F. for atleast four hours before converting into a foam and coagulating to formsponge rubber in the usual manner. The treatment with formaldehyde takesplace befor converting the compounded latex into a foam, and before orafter the addition of the filler material to the latex.

The improvement of the present invention may be obtained with laticespreserved in various manners, for example, latex preserved with ammoniaalone, or latices preserved with ammonia and a small amount of agermicidal agent, such as formaldehyde, hydroxylamine, alkyl mercuryester, arsenic oxide, sodium pentachlorphenate, or the like. Five-tenthspart of aldehyde, such as formaldehyde, is sufficient to produce theimprovement on heating of the latex, but larger amounts of formaldehydemay be-used. From the standpoint of increasing the wet coagulum strengthand the flexing characteristics of the sponge, there does not appear tobe any advantage in using more than 1 part of formaldehyde per 100 partsof latex solids. It is preferred to maintain a slight excess of ammoniaover that necessary to react with the added formaldehyde during theheating of the latex. The latex should be heated to above 100 F. for atleast 4 hours. A temperature between 100 and 120 F. is satisfactoryalthough higher temperatures may be used if the latex is not undulydestabilized thereby. The treatment with formaldehyde at elevatedtemperatures should be for at least 4 hours and may readily be continuedfor 24, or even 48 or more hours, if-desired. Various filler materialswhich have been used with latices and which have shown the improvedproperties according to the present invention include barytes, clay,whiting, ground silica, and the like.

The example below illustrates the process of the present invention. Acompound of the following formula was prepared:

Parts by weight Latex:

Rubber 100 Water 60 Ammonia .3

Formaldehyde (as 37% aqueous solution)- 1 Potassium oleate (as 20%aqueous solution) .5 Sodium hydroxide (as aqueous solution) .05

The latex used was ammonia preserved Hevea latex which had beenconcentrated by centrifuging and which contained .65% ammonia. Theammonia content was reduced to .3% by aeration. After theaddition of theformaldehyde, the alkalinity of the latex in terms of ammonia was .1%.The potassium oleate and sodium hydroxide acted as stabilizers for thelatex during the heating. The latex was placed in a suitable containerand heated at 110 F. for 12 hours, after which it was allowed to come toroom temperature. After this treatment there was added to the latex, forevery 100 parts by weight of dry rubber content, a compositioncomprising 7 parts by weight (wet) of a 35% aqueous solution of castoroil soap, 100 parts by weight (wet) of a 50% aqueous paste of barytes, 4parts by weight (wet) of a 60% aqueous paste of sulphur, .7 part (wet)of a 50% aqueous paste of zinc diethyl dithiocarbamate, and .5 part(wet) of a 50% aqueous paste of zinc mercapto benzothiazol. The castoroil soap is a foaming agent, and the last two named compoundsaccelerators. The compounded latex was then placed in an electricallydriven cake mixer and air was beaten into the compound until the desiredfoam density was obtained. There was then thoroughly stirred into thefoam 16 parts (wet) of a 50% aqueous paste of zinc oxide and 4 parts(wet) of a 50% aqueous pasteof sodium silico fluoride for every 100parts of dry rubber content. The foam was then poured into suitablyshaped molds and allowed to gel at room temperature (73 B). At 73 F. thefoam gelled in about 8 minutes. The gelled spongewhile still in themolds was vulcanized in air at 250 F. for minutes. After vulcanizationthe wet sponges were stripped from the mold without difllculty,dewatered in a centrifugal drier, and dried in a current of dry air at160 F. for 14 hours. The final sponge rubber product had a definitelyhigher flexing resistance than similarly compounded sponge without thepreliminary treatment of the latex with formaldehyde at elevatedtemperature.

In view of the many changes and modifications that may be made withoutdeparting from the principles underlying the invention, reference shouldbe made to the appended claims for an understanding of the scope of theinvention.

Having thus described my invention, what I claim and desire to protectby Letters Patent is:

1. The method of making sponge rubber which comprises adding to latexcontaining ammonia at least .5 part of formaldehyde per parts of rubbersolids of the latex but insufficient to react with all the ammoniapresent, and heating without coagulation at a temperature above 100 F.for at least 4 hours and compounding the latex with at least 10 partsfiller material per 100 parts of rubber solids of the latex, convertingthe compounded latex into a foam, and coagulating the latex in the foamto form sponge rubber.

2. The method of making sponge rubber which comprises adding to latex atleast .5 part of formaldehyde per 100 parts of rubber solids of thelatex and heating in the presence of a stabilizer and withoutcoagulation at a temperature above 100 F. for at least 4 hours andcompounding the latex with at least 10 parts filler material per 100parts of rubber solids of the latex, converting the compounded latexinto a foam, and coagulating the latex in the foam to form spongerubber.

3. The method of making sponge rubber which comprises adding to latexcontaining ammonia at least .5 part of formaldehyde per 100 parts ofrubber solids of the latex but insufiicient to react with all theammonia present, and heating without coagulation at a temperature above100 F. for at least 4 hours and compounding the latex with at least 10parts filler material per 100 parts of rubber solids of the latex,beating the compounded latex into a foam, and coagulating the latex inthe foam to form sponge rubber.

4. The method of making sponge rubber which comprises adding to latex atleast .5 partof formaldehyde per 100 parts of rubber solids of the latexand heating in the presence of a stabilizer and without coagulation at atemperature above 100 F. for at least 4 hours and compoundin the latexwith at least 10 parts filler material per 100 parts of rubber solids ofthe latex, beating the compounded latex into a foam, and coagulating thelatex in the foam to form sponge rubber.

WILLIAM J. CLAYTON.

